Titanium-base alloys



Nov. 17, 1970 c. D. T. MINTQN EI'AL wmmg TITANIUM-BASE ALLOYS Filed April 28. 1969 BASE HEAT TREATMENT. 9 HQUQ 825QGQ-T- 24- HQURS jiw/f/waes 2M6 M15) fiaMmsMA/m v 5019a? [EA fir $44.52?

wwm M Int. Cl. 022C 27/00 U.S. Cl. 148-325 8 Claims ABSTRACT OF THE DISCLOSURE A titanium-base alloy having very high ultimate tensile strength, a high ratio of notched tensile strength to ultimate tensile strength and good forgeability containing by weight 2.5-7.5% aluminium, 27% zirconium, 2.55.5% molybdenum, O.5-1.75% copper, 0.11.0% silicon and optionally ODS-0.4% carbon and up to 7% tin.

This invention relates to titanium-base alloys and is a continuation-in-part of our prior application Ser. No. 514,364, filed Dec. 16, 1965, now abandoned,

BACKGROUND OF THE INVENTION Titanium alloys, because of their favourable combination of strength and density are used in the construction of aircraft components and in other applications in which good strength to weight ratio is important. Among the properties which are also desirable in such alloys is that of forgeability, since many components require to be shaped by forging.

Experience in the application of titanium alloys has led to the appreciation of the importance of the property of notch strength which has not until recently been considered to be of prime importance. Notch strength is the stress to fracture a specimen having a notch machined in its surface, and in practical terms the importance of the notch is that it acts as a stress raiser and shows whether the alloy is susceptible or not to fracture where there is a sudden change of cross-section in the component under stress.

Usually an alloy having very high ultimate tensile strength has low notch strength. Thus it is possible for an alloy to possess high tensile strength and yet have low notch strength. Such an alloy, if selected for use on the basis of tensile strength, may fail in service if its shape introduced a stress raiser. In order for such a component to operate safely, it is necessary, not only for the tensile strength to be adequate, but also the notch strength to be high. The ratio of notched tensile strength to ultimate tensile strength is a measure of the toughness of an alloy.

The present invention has as an object the provision of a titanium-base alloy having high notch strength and very high ultimate strength.

A further object is the provision of an alloy which is tough, forgeable and ductile.

3,54%,946 Patented Nov. 17, 1970 ice Another object of the invention is the combination in one alloy of the properties of high tensile strength and high notch strength, together with low density whereby a high strength to weight ratio is achieved.

SUMMARY OF THE INVENTION According to the invention, a titanium-base alloy possessing very high ultimate strength, high notch strength and good forgeability consists essentially of, by weight, aluminium 2.57.5%, zirconium 2-7 molybdenum 2.5- 5.5%, copper 0.51.75%, silicon (Ll-1.0%, balance titanium, apart from usual impurities.

The alloy may also contain O.50.4% carbon.

A preferred range of composition is 5.56.5% aluminium, 4-6% zirconium, 3.5-4.5 molybdenum, 0.1-0.5 silicon, 0.75-1.25% copper, 0.05-0.1% carbon, balance, apart from usual impurities, titanium.

Aluminium and zirconium are elements which stabilise the alpha phase of titanium and function as solid solution strengtheners. The elements molybdenum and copper stabilise and strengthen the beta phase.

In alloys according to the invention, the optimum combination of high strength and good ductility is obtained by working the alloys below the alpha plus beta/beta transformation temperature, that is at temperatures at which the alloys consist of a mixture of alpha and beta phases.

Alloys containing only alpha stabilising elements are difficult to work in the alpha plus beta field. In alloys according to the invention the addition of beta stabilisers, molybdenum and copper, improves forgeability in the alpha plus beta field whilst also strengthening the alloys.

The silicon content of the alloy is preferably in the range 0.10.5%. This element is a compound former which strengthens the alloy when heat-treated and the effects of various amounts of silicon on the tensile properties of an alloy in accordance with the invention are shown in Table I. It will be seen that 0.25% brings about a marked increase in strength.

Optionally, an alloy in accordance with the invention may contain 0.050.4% by weight of carbon whereby the strength/ weight relationship is improved. Carbon is a potent alpha stabiliser which considerably raises the alpha plus beta/beta phase transition temperature (transus). For example, 0.1% carbon raises the transus by C. compared with a carbon content at impurity level. This increase enlarges the alpha plus beta field and permits forging to be carried out in the alpha plus beta field at a higher temperature, thereby reducing the resistance to deformation. The addition of carbon also increases the strength of alloys of the invention without significant loss of ductility.

The aluminium is the principle alpha stabiliser and the content of this element may be modified by the presence of tin in replacement for aluminium in a known manner in the ratio of 3% tin for each 1% aluminium replaced, up to a maximum of 7% tin, provided that the solid solution hardening effect is equivalent to that of 2.57.5% aluminium (aluminium equivalent). The best range of aluminium or aluminium equivalent content is 5.5 6.5% (optimum 6) and this range, combined with about 5% zirconium, produces the optimum combination of strength and ductility. Tin has the disadvantage of having a greater density than aluminium and a tin-free alloy is preferred for aircraft applications.

The invention is based on the discovery that the presence of certain beta stabilising elements produces a twofold effect, namely high tensile strength and high notch strength. It is, of course, known that beta stabilisers increase tensile strength, but the effects on notch strength are less well understood. From our study of the effects of two beta stabilisers, one a beta-isomorphous element, molybdenum, and the other a beta-eutectoid element, copper, it is apparent that within a small range of content of these elements, the notch strength can be raised to a very high level. Excessive copper leads to inferior notched tensile strength, and investigations using electron microscopy suggest that this is due to the formation of a fine precipitate of copper compounds. Tensile strength is raised to a high level over a much wider range of composition, but the combination of very high notch strength and high tensile strength is found only in a limited range of combined molybdenum and copper content. Very high notched tensile strengths in the range 117 to 125 tons/in. (262,000 to 280,000 lb./in. are obtained in the range of 45% molybdenum and 0.75-1.25 copper.

A modern design criterion for aircraft demands a ratio of notched tensile strength to ultimate tensile strength of 1.25 at a theoretical stress concentration factor of 3.0 and the present invention provides an alloy capable of reaching this standard and which possesses an ultimate tensile strength in excess of 85 tons/in. (190,000 1b./in. after suitable heat-treatment. No other alloy is known which meets these two capabilities. Table II shows the tensile properties of alloys having a basic composition of 6% aluminium, zirconium, 0.2% silicon, 0.1% carbon and containing various amounts of molybdenum and copper. Comparison of the data in the table shows that alloys 2, 4, 8a, 8b, 9, 11 and 13 have tensile strengths in excess of 85 tons/in. and a ratio of notched tensile strength to ultimate tensile strength (NTS/UTS) of 1.25 or more, which is a very high ratio. The above mentioned alloys have compositions which show that the critical range for achieving such high ratios is 2.5-5.5% molybdenum and 0.5-1.75% copper, and of these ranges the preferred composition is 4% molybdenum and 1% copper.

It will be noted from Table I that examples of alloys containing molybdenum or copper just outside the broad ranges of composition herein disclosed show that the NTS/UTS ratio or the ultimate tensile strengths fall away at the terminal compositions. At the same time, the important property of ductility has to be considered and it will be observed that when copper is outside the maximum limit, with more than about 3% molybdenum, as in alloys 5 and 14, both the ductility and the NTS/UTS ratio are low. In Table III are given further examples of tensile strengths obtainable with various molybdenum and copper contents.

BRIEF DESCRIPTION OF THE DRAWING The figure is a portion of a constitutional diagram of an alloy containing as a base 6% aluminium, 5% zirconium, 0.2% silicon and 0.07% carbon showing the effect of the molybdenum and copper contents. The lines 80T and 100T are lines of constant strength representing ultimate tensile strengths of 8 0 and 100 tons/in. (179,000 and 224,000 1b./in. respectively. A number of points are shown with the tensile strength in tons/in. and, in brackets, the NTS/UTS ratio indicated alongside and the shaded area represents the compositions having ultimate strengths in excess of 85 tons/in. (190,000 lb./in. and a NTS/UTS ratio of 1.25 or more. These points are based on Table II. The shaded area corresponds approximately to the parallelogram formed by the compositions, 0.5 and 1.75% copper and 2.5 and 4 5.5% molybdenum, the alloys being heat-trated at 825 C. for one hour, oil quenched and aged 24 hours at 500 C.

Examples of the results obtainable by various heattreatments are given in Table IV. The plain strength (particularly UTS) increases as the solution temperature increases and as the subsequent cooling rate increases. The NTS/UTS as might be expected decreases and the plain strength increases. Slow cooling from a high temperature (900 C.) by furnace cooling, whilst maintaining the NTS/UTS ratio, produces a low tensile strength. The only heat-treatments producing adequate ultimate tensile strength and notch strength in these sections are oil quenching from 825 C. or air cooling from 900 C. with the preferred composition. It may be desirable to quench from a temperature in the range 825850 C. for optimum properties in thicker sections.

In addition to the high NTS/UTS ratio and good tensile strength, alloys in accordance with the invention have good forgeability which makes them easy to shape. In Table V, the resistance to deformation as determined by plastometer on specimens initially 0.4 in. diameter and height, is shown for two alloys outside the scope of the present invention and for the preferred composition. The alloys containing 2% molybdenum and 2% copper are just outside the range of the present invention. Of these latter two alloys, one is also lacking in carbon to show the effect of this element on forgeability. There is a considerable drop in resistance for deformation as a result of the modified molybdenum and copper contents.

The combination of elements according to the invention has the further advantage in that the density of the best alloy has a density only slightly greater than that of pure titanium and this gives a particularly favourable strength/Weight ratio. The strength of alloy is similar to that of many heat-treated steels but the weight is little more than half and this makes the alloy particularly suit able for use for moving parts.

A further advantage of the alloys in accordance with the invention is that they also possess useful elevated temperature properties. The tensile properties at 400 C. of /2 in. thick cheese are: 0.1% offset yield, 55.7 tons/ in. (125,000 lb./in. 0.2% offset yield, 57.2 tons/in. (128,000 1b./in. ultimate tensile strength 69.8 tons/in. (156,000 lb./in. elongation on 4 /;11 /2% and reduction in area 28%. Total plastic creep strain in hours under a stress of 37 tons/in. (82,900 lb./in. is 0.15%. These properties are obtained on alloys of the preferred composition, oil quenched from 825 C. and aged 24 hours at 500 C.

Alloys according to the invention may, with advantage, be used for the manufacture of steam and gas turbine components, such as discs and blades, because of their suitable combination of strength, notch toughness, ductility, creep resistance, density, forgeability and corrosion and erosion resistance. Another important use is for airframe components, such as parts for the under-carriage.

The alloys may also be used with advantage for the manufacture of end bells for alternators by reason of the good strength to weight ratio combined with nonmagnetic properties.

TABLE I.EFFEC'I OF SILICON ON PROPERTIES Nora-V inch diameter rod heat-treated 1 hour at 010 0., oil quenched, aged 24 hours at 500 C. and an cooled.

TABLE IL-TENSILE PROPERTIES OF 6A l-5Zr-M0-Ou0.2Si-0.07C TYPE ALLO YS Percent Percent 0.1% PS, UIS P n RA, NTS NTS/ Alloy N0. M Cu tons/in? tons/in. 4 /8 percent tons/in. UTS

* Notched tensile strength with a theoretical stress concentration factor of 3.0. 0.5 in

diameter rod produced from 150 and aged 24 hours at 500 0. AC.

g. ingots, heat-treated for 1 hour at 825 0., oil quenched NoteBar of cross-section 1 Inch by 3 inches, heat-treated lihour at 910 0., oil quenched,

aged 24 hours at 500 C. and air cooled.

TABLE IV.EFFECT OF HEAT-TREATMENT ON THE TENSILE PROPERTIES OF THE PREFERRED COMPOSITION [Compositiom 6Al-5Zr-4Mo-1.45Cu-0.2Si-0.070 (density at 20 0.: 4.58 g./cm.

0.1 E1 Heat- Ps, UTS, P n RA, NIS, NTS/ Material treatment+ tons/in. tons/in. 4 /80 percent tons/in. UTS

1 diameter rod (A6012) 9000Q 4 1 5- 0 9 28 118 1. 12 82501;), 80. 7 94. 2 12 33 122 1. 30 QOOAC 80. 8 85. 8 18 39 119 1. 39 QOOFC 74. 4 76. 0 21 43 112 1. 47 1" thick cheese (A6012) 850OQ 81. 0 96- 2 13 17 105 1. 09 825002 81. 7 89. 6 13 26 120 1. 34

+900 0Q=1 hour 900 0. oil quenched and aged 24 hours 500 C. air cooled; 850 001-1 hour 850=C. oil quenched and aged 24 hours 500 0. air cooled; 900 AC-l hour 900 C. arr cooled and aged 24 hours 500 C. air cooled; 900 FC=1 hour 900 C. furnace cooled to 500 C. arr cooled and aged 24 hours 500 0. air cool.

NTS=Notched tensile strength with a theoretical stress concentration factor of 3.0.

TABLE V.-FORGEABILITY OF VARIOUS ALLOYS Maximum resistan/oe E0 deformation 1. A titanium-base alloy in the solution heat treatedaged condition having an ultimate tensile strength in excess of 190,000 1b./in. a ratio of notched tensile strength to ultimate tensile strength of at least 1.25 at a stress concentration factor of 3 and good forgeability, said alloy consisting essentially of, by weight, 2.5-7.5 aluminum, 27% zirconium, 2.5-5.5 molybdenum, 0.5-1.75% copper, 0.1-1.0% silicon, balance titanium apart from impurities.

2. A titanium-base alloy as claimed in claim 1 containing ODS-0.4% carbon.

3. A titanium-base alloy as claimed in claim 1 in which aluminium is replaced by tin up to a maximum of 7% tin in the portion of 3% tin for each 1% aluminium replaced, the combined alpha strengthening of tin and aluminium being equivalent to that of 2.5-7.'5% aluminium.

4. A titanium-base alloy as claimed in claim 3 containing ODS-0.4% carbon.

5. A titanium-base alloy as claimed in claim 3 in which tin and aluminium are present in an amount such that the combined alpha strengthening effect is equivalent to that of 6% aluminium.

6. A titanium-base alloy in the solution heat treatedaged condition having a tensile strength in excess of 190,- 000 lb./in. and a ratio of notched tensile strength to ultimate tensile strength of at least 1.25 at a stress concentration factor of 3, with good forgeability, said alloy consisting essentially of by weight 5.56.5% aluminium, 4-6% zirconium, 2.55.5% molybdenum, 0.51.75% copper and 0.1-0.5% silicon, 0.05-0.15% carbon, balance titanium, apart from usual impurities.

7. A titanium-base alloy as claimed in claim 1 containing 3.5-4.5% molybdenum and 0.75-1.25% copper, said alloys possessing notched tensile strengths of 262,000 to 280,000 1b./in. with a stress concentration factor of 3.

8. A titanium-base alloy of claim 6 consisting essentially of 6% aluminium, 5% Zirconium, 4% molybdenum, 1% copper, 0.2% silicon, 0.07% carbon, balance titanium and impurities.

References Cited UNITED STATES PATENTS 2,892,864 7/1959 Harris et a1. 175.5 3,049,425 8/ 1962 Fentiman et a1. 75-175.5 3,105,759 10/1963 Fentiman et al 75175.5 3,378,368 4/1968 Minton et al. 75-175.5

CHARLES N. LOVELL, Primary Examiner US. Cl. X.R. 75175.5 

